Emerging roles for protein histidine phosphorylation in cellular signal transduction: lessons from the islet β‐cell

Introduction Protein histidine phosphorylation in prokaryotes and eukaryotes Identification and characterization of histidine kinases in the pancreatic β‐cell A membrane‐associated histidine kinase phosphorylates the G β ‐subunit of trimeric G‐proteins A novel histone H4‐phosphorylating histidine kinase in islets β‐cells Regulation of protein histidine phosphorylation in islet β‐cells Functional consequences of protein histidine phosphorylation Regulation of the mitochondrial function via protein histidine phosphorylation Regulation of ion channels Regulation of isoprenoid metabolism Protein histidine phosphatases • Potential defects in histidine phosphorylation in islets derived from the Goto‐Kakizaki rat, a model for type 2 diabetes Conclusions and future research Abstract Protein phosphorylation represents one of the key regulatory events in physiological insulin secretion from the islet β‐cell. In this context, several classes of protein kinases ( e.g. calcium‐, cyclic nucleotide‐ and phospholipid‐dependent protein kinases and tyrosine kinases) have been characterized in the β‐cell. The majority of phosphorylated amino acids identified include phosphoserine, phosphothreonine and phosphotyrosine. Protein histidine phosphorylation has been implicated in the prokaryotic and eukaryotic cellular signal transduction. Most notably, phoshohistidine accounts for 6% of total protein phosphorylation in eukaryotes, which makes it nearly 100‐fold more abundant than phosphotyrosine, but less abundant than phosphoserine and phosphothreonine. However, very little is known about the number of proteins with phosphohistidines, since they are highly labile and are rapidly lost during phosphoamino acid identification under standard experimental conditions. The overall objectives of this review are to: ( i ) summarize the existing evidence indicating the subcellular distribution and characterization of various histidine kinases in the islet β‐cell, ( ii ) describe evidence for functional regulation of these kinases by agonists of insulin secretion, ( iii ) present a working model to implicate novel regulatory roles for histidine kinases in the receptor‐independent activation, by glucose, of G‐proteins endogenous to the β‐cell, ( iv ) summarize evidence supporting the localization of protein histidine phosphatases in the islet β‐cell and ( v ) highlight experimental evidence suggesting potential defects in the histidine kinase signalling cascade in islets derived from the Goto‐Kakizaki (GK) rat, a model for type 2 diabetes. Potential avenues for future research to further decipher regulatory roles for protein histidine phosphorylation in physiological insulin secretion are also discussed.